Novel Concept of Experimental Setup for Characterisation of Plastic Yielding of Sheet Metal at Elevated Temperatures

Abstract

In times of highest significance of process modelling and numerical simulation characterisation of material properties is of special importance for tools’ and components’ dimensioning. But in general material properties depend on many different influencing variables, e.g. temperature, humidity and many others. Especially in fields of sheet metal forming the mechanical behaviour of components highly differs according to real stress condition. In particular yield loci combine the information of beginning of yielding with a biaxial stress condition, but nevertheless for many materials they have not been determined yet. For all others the existing values are available only at room temperature. In this paper a novel concept of the experimental setup is shown, with which plastic yielding of sheet metal can be examined also at elevated temperatures. In usual biaxial tension tests cruciform specimen are drawn in plane. The new machine-concept, which is presented in this paper, is based on a punch-load moving perpendicular to the sheet. By clamping the specimen restoring forces are induced, which cause in dependence of special developed tool and work piece geometries defined stress conditions. Using an optical measurement system for determination of strains with CCDcameras of very high frame rate allows exact identification of starting plastification by offline analysis. Experiments at elevated temperatures are realised by local heating with a diode laser and a special optical system to reach a homogenous distribution of temperatures in the forming zone. On the one hand these investigations are necessary for many materials to achieve further information on characteristic properties in warm forming, because their data are only known at room temperature. On the other hand some materials, e.g. magnesium wrought alloys, are mostly formed at elevated temperatures (here in the range of 200°C to 250°C), because of its significant higher formability. Thus, material behaviour must be characterised at these temperatures.

In-plane biaxial compression and tension testing of thin sheet materials

“…There are geometries where slits are included, presented in Demmerle and Boehler (1993), Green et al (2004) and Merklein and Biasutti (2013). Geometries for thinner sheet metals are presented in Kreißig and Schindler (1986), Müller and Pöhlandt (1996) and Geiger et al (2005) and a general overview is given in Kuwabara (2007) and Hannon and Tiernan (2008). As in our work only thin sheet metal is tested and a certain amount of plastic strain in the center region is needed, a design referring to the ones presented in Kreißig and Schindler (1986) as well in Müller and Pöhlandt (1996) was chosen.…”

Experimental determination of yield loci for magnesium alloy AZ31 under biaxial tensile stress conditions at elevated temperatures

“…The results that are presented in this paper have been obtained from experiments done with the LFT-setup which is in general described in [15], but modified concerning some, but nevertheless very important details. The main aspect is the measurement of the reaction forces in rolling direction (RD) and in transverse direction (TD) of the cruciform sheet, since they are essential for the determination of the yield stresses as it is shown later on.…”

“…By the usage of a special device moving perpendicular to the sheet, reaction forces are caused. The first one was introduced by Geiger et al in 2005 [15], first experimental results of experimental investigations for AZ31 in a temperature range from 20 up to 310°C are given in [16]. In 2007 Ghiotti et al [17] proposed another setup, where inductive heating is applied to realize maximum temperatures in a range of 750°C, but no experimental results concerning yield loci determined at these temperatures have been published until today.…”

Characterization of yielding behavior of sheet metal under biaxial stress condition at elevated temperatures

“…For the experimental determination of the onset of yielding for various biaxial stress states a novel experimental setup has been developed at the Chair of Manufacturing Technology (LFT) at the University of Erlangen [10] and the setup as well as the evaluation methodology is patent-protected [11]. It is characterized by a compact design, so that additional features, e.g.…”